A number of optical gas detection techniques exist and are based on the measurement of the absorption of incident radiation by the gas molecules. For example, non-dispersive infrared (NDIR) spectroscopy involves using a detector to monitor radiation transmitted by a sample when exposed to a radiation source. By measuring the radiation absorbed by the sample within a particular wavelength range, the concentration of a target gas in the sample can be determined.
As explained in WO-A-01/65219 and WO-A-01/40748, another known type of optical absorption gas detector is the correlated interference polarisation spectrometer (CIPS). A CIPS works on the principle that for any given wavelength incident radiation, the quantity that is absorbed by the gas is a function of the cross-section (σ(λ)) of any particular molecule of the gas. If the dependence of the cross-section on the wavelength is very pronounced, then the gas molecules will absorb radiation over a very narrow waveband. Accordingly, the spectral displacement between the maximum and minimum intensities of the transmitted radiation is very small. By measuring the difference between the maximum and minimum intensities of the transmitted radiation, it is possible to calculate the concentration of the gas.
The CIPS uses the quasi-periodical structure of the electronic absorption spectrum of the gas molecules, which occurs due to their vibrational-rotational properties. FIG. 1 shows, as an example, the absorption spectrum of methane in the vicinity of 3.25 micrometers. The spectrum consists of a number of very narrow (approximately 1 nm wide) quasi-periodic absorption bands, labelled 2, that are detected by the CIPS.
In order to detect the quasi-periodic structure, the CIPS filters radiation transmitted by the sample gas using a comb filter which is generated by a controlled interference polarisation filter (cIPF). The cIPF is formed from a modified interference polarisation filter (IPF) which uses the phenomenon of birefringence in certain crystals to obtain a transmission spectrum which is characterised by a quasi-periodic sequence of spectral passbands.
In order to be able to use the IPF in the detection of gases, the IPF must provide a transmission spectrum that closely matches the quasi-periodic absorption spectra of the gas to be detected (i.e. the bandwidth between adjacent peaks in the absorption spectrum of the gas to be detected must correspond to the bandwidth between adjacent transmission peaks in the IPF transmission spectrum). Furthermore, the IPF must be able to shift this spectrum in time so that one can detect the intensity of the radiation transmitted at both the absorption and non-absorption bands of the absorption spectrum of the target gas. Various ways of achieving this are disclosed in WO-A-01/65219.
It will be appreciated that, whilst this description will generally refer to the detection of gases, the techniques and apparatus disclosed herein could be used for the measurement of any substance.
One problem frequently encountered in many types of optical absorption spectrometer, including NDIR devices and CIPS apparatus, is that the measurement range of the instrument is inherently limited. For example, a CIPS of the sort described in WO-A-01/65219 and WO-A-01/4702 has a measurement range typically limited to three or four orders of magnitude. For concentrations below its desired range, the CIPS does not have the sensitivity (i.e. signal/noise ratio) to measure the concentration accurately. For concentrations above its designed range, the signal output by the CIPS saturates and will not respond properly.
One factor which contributes to this problem is the design of the cavity used to contain the gas sample during test. For high sensitivity, a long gas cavity having for example an optical path between 20 cm and 30 cm is required in order to ensure that there is a sufficient number of gas molecules in the chamber to achieve detectable absorption at low gas concentration levels and obtain a reasonable signal in the presence of noise. The disadvantage of using a long optical path is that, in the presence of higher gas concentrations, substantially all the radiation is absorbed by the gas molecules before it can reach the detector (in accordance with Beer's law). As such, it is not presently possible to use a single apparatus for the accurate measurement of high gas concentrations as well as low gas concentrations.
The problem is also made worse by the dependence of the measured radiation signal on uncontrollable measurement conditions. For example, factors such as dust occurring in the optical system or radiation beam misalignment, etc., can decrease the incident radiation intensity, leading to an erroneous concentration measurement.
What is needed is an optical absorption spectrometer which can operate across a wide range of target gas concentrations. It would also be advantageous is the measurement signal were less effected by changes in the lighting conditions.